The present invention relates to a non-aqueous electrolytic secondary battery. More particularly, the present invention relates to an improvement in a lithium ion secondary battery, specifically, a positive electrode active material capable of improving charge and discharge cycle characteristic and safety of lithium ion secondary battery, a positive electrode active material composition, and to a lithium ion secondary battery having improved charge and discharge cycle characteristic, low temperature characteristic and safety.
Lithium ion secondary batteries have superior electromotive force and battery capacity, and are more advantageous than nickel-cadmium battery etc. in that they show high energy density, high voltage etc. While they have been drawing much attention in recent years, they have been more often employed as driving force of portable devices, such as cellular phones and note type personal computers. Motivated by the situation as described, various studies have been undertaken in the pertinent field to provide a higher performance product. To be specific, such studies focus on the properties and preferable combinations of the constituent materials of the Lithium ion secondary batteries: positive electrode active materials, negative electrode active materials, electrolytes and the like.
As a positive electrode active material for lithium ion secondary batteries, there have been proposed a number of Li-transition metal composite oxides, such as Lixe2x80x94Mn type composite oxide, Lixe2x80x94Ni type composite oxide, Lixe2x80x94Co type composite oxide and the like. Of these, Lixe2x80x94Co type composite oxides have been predominantly put into practice, because they are chemically stable, can be handled easily and are capable of producing secondary batteries having high capacity. There are many suggestions and reports to further improve Lixe2x80x94Co type composite oxides to ultimately improve the properties of the secondary battery that uses a Lixe2x80x94Co type composite oxide as a positive electrode active material. For example, JP-B-7-118318 discloses that LiCoO2 can improve the discharge capacity of a secondary battery, which LiCoO2 is obtained by adding a rich amount of a lithium compound and a cobalt compound, heating the compounds and removing, by washing with water, unreacted lithium compound and lithium carbonate byproduct in the reaction product.
Mostly, the positive electrode active material for a lithium ion secondary battery is a layer made from a composition (hereinafter to be also referred to as a positive electrode active material composition) consisting of a conductive material and a binder made from an organic polymer. As the conductive material, various graphites and carbon black are used. The positive electrode active material is generally used in the form of particles, each particle dispersed in a non-conductive binder in the composition. Absence of a conductive material leads to an electrically insulated state of each particle in the positive electrode active material due to the action of the binder, which in turn makes the layer of the positive electrode active material composition (hereinafter to be also referred to as a positive electrode active material layer) substantially electrically insulating. The conductive material is used to make this layer conductive by its presence between the particles of the positive electrode active material to electrically connect the particles. Consequently, the positive electrode active material layer as a whole becomes conductive. The positive electrode active material is used in the form of particles as mentioned earlier. When the particle size is too small, the reactivity during charge and discharge of a secondary battery sometimes becomes too great to the extent that abnormal cell reaction is induced to a dangerous level. The present inventors have found that, from the aspect of the safety of the secondary battery, the preferable average particle size of a positive electrode active material is not less than 10 xcexcm. However, an average particle size of not less than 10 xcexcm lowers the conductivity of the positive electrode active material layer, frequently causing degraded charge and discharge cycle characteristic.
With regard to the negative electrode active material and electrolyte, for example, JP-A-6-36802 discloses that charge and discharge cycle characteristic of a lithium ion secondary battery can be improved by using a positive electrode active material made from a Li-transition metal composite oxide, a negative electrode active material made from a specific pitch type carbon fiber, and a mixed solvent of one or more members from a group of ethylene carbonate, propylene carbonate, butylene carbonate, xcex3-butyrolactone, sulfolane, 3-methylsulfolane, tetrahydrofuran, 2-methyltetrahydrofuran, acetonitrile, dimethoxyethane, diethoxyethane, dimethylsulfoxide, dioxolane, 4-methyldioxolane and diethyl carbonate, as an electrolyte. U.S. Pat. No. 5,561,005 discloses that charge and discharge cycle characteristic of a lithium ion secondary battery can be improved by using a positive electrode active material made from a Li-transition metal composite oxide, a graphite type carbon material as a negative electrode active material, and a mixed solvent of ethylmethyl carbonate and dimethyl carbonate as an electrolyte, and further a mixture of this mixed solvent and ethylene carbonate or propylene carbonate as an additional component. When such component is to be added, it should be noted that addition of propylene carbonate to a graphite type carbon material as the negative electrode active material results in the decomposition of the solvent, and in this case, therefore, ethylene carbonate is preferably used. When the negative electrode active material is a carbon material other than graphite, propylene carbonate is preferably used (see paragraph 59).
As mentioned above, lithium ion secondary battery has many superior characteristics as compared to nickel-cadmium battery etc., in high energy density, high voltage and the like. On the other hand, it has unpreferable low temperature characteristic in that discharge at a low temperature results in lower discharge capacity and lower discharge voltage than in the case of discharge at room temperature. Particularly, at an extremely low temperature of not more than xe2x88x9220xc2x0 C. (and not more than xe2x88x9235xc2x0 C.), the discharge voltage shows a sharp drop at the initial stage of discharge. In the discharge curve [axis of abscissas: discharge capacity rate (%), axis of ordinate: discharge voltage (V) ] of lithium ion secondary battery at a low temperature, the minimum value and the maximum value sequentially appear in the direction of increase of the discharge capacity rate. However, at an extremely low temperature of not more than xe2x88x9220xc2x0 C., the difference between the minimum value and the maximum value becomes about 0.3 (V)-0.5 (V) and the difference between the minimum value and the discharge voltage, when the discharge capacity rate is 0%, becomes about 0.5 (V)-1.0 (V). This causes a drastic drop of voltage at the initial stage of discharge. In effect, the discharge voltage could fall under the stop voltage set for each equipment, thereby making the equipment practically unoperatable. This problem of low temperature characteristic prevents application of the battery to observation monitors, communication devices, electric automobiles, electric power reservoir and the like, that may be used in frigid places. While some measure for preventing the battery from falling to a temperature below a certain level by, for example, placing the battery in a warmer place or applying a heater, will enable use of the battery for the above-mentioned equipment, this results in an increased cost of the equipment. Therefore, the major problem of the lithium ion secondary battery is to overcome the low temperature characteristic. Lithium ion secondary battery typically has a structure wherein a positive electrode sheet and a negative electrode sheet are opposed via a separator, and an electrolyte fills the gap between the both electrodes. The positive (negative) electrode is produced by forming a positive (negative) electrode active material layer made from a positive (negative) electrode active material, a conductive material and a binder on a current collector such as a metal foil. The positive electrode active material and the negative electrode active material are particulate as mentioned above, including gaps due to the particles. This allows the electrolyte to pass through the gap into the electrode, where chemical change occurs for charge and discharge. More active chemical change in the electrode enables improvement in the battery capacity, rate characteristic, and low temperature characteristic. A sufficient gap may be secured by enlarging the particles of the active material, but a greater particle size reduces the filling density of the active material and battery capacity per volume. An electrolyte that does not show an increased viscosity at a low temperature is considered to improve low temperature characteristic, because it allows penetration of electrolyte into the electrode at a low temperature without decreasing the density of the active material. In conventional electrolytes, however, a lower viscosity is associated with a higher freezing point. For example, dimethyl carbonate, which is among the components typically added to an electrolyte, lowers the viscosity of an electrolyte when added in greater amounts, but conversely raises the freezing point of the electrolyte.
It is therefore an object of the present invention to provide an improved Li-transition metal composite oxide that is used as a positive electrode active material of a non-aqueous electrolyte secondary battery, particularly lithium ion secondary battery, and that is capable of improving charge and discharge cycle characteristic of a battery, as well as a lithium ion secondary battery containing the Li-transition metal composite oxide as a positive electrode active material (a first lithium ion secondary battery).
The present invention also aims at providing a positive electrode active material composition having an improved conductivity, which contains a positive electrode active material (Li-transition metal composite oxide) having an average particle size of not less than 10 xcexcm, which is preferable for the safety of the secondary battery, as well as a lithium ion secondary battery containing this positive electrode active material composition (a second lithium ion secondary battery).
The present invention further aims at providing a lithium ion secondary battery noticeably improved in charge and discharge cycle characteristic due to the combination of a novel negative electrode active material capable of improving charge and discharge cycle characteristic, and an electrolyte (a third lithium ion secondary battery).
The present invention yet aims at providing a lithium ion secondary battery having sufficiently improved charge and discharge cycle characteristic, storage characteristic and low temperature characteristic, without decreasing energy density (a fourth lithium ion secondary battery).
Further, the present invention aims at providing a lithium ion secondary battery free of reduction in the discharge capacity and discharge voltage at a low temperature (not more than xe2x88x9220xc2x0 C., particularly not more than xe2x88x9235xc2x0 C.), which is attributable to the use of a specific electrolyte having a low viscosity, and therefore, free of solidification under low temperature (a fifth lithium ion secondary battery). The present invention further aims at providing a lithium ion secondary battery free of a sharp drop of the discharge voltage at the initial stage of discharge under an extremely low temperature, particularly not more than xe2x88x9220xc2x0 C. (a sixth lithium ion secondary battery).
To achieve the above-mentioned objects, the present invention has the following characteristics.
In a first aspect of the present invention, a particulate Li-transition metal composite oxide having an average particle size of not less than 10 xcexcm, wherein [20/(specific surface areaxc3x97average particle size) ]=7-9, is used as a positive electrode active material, thereby to improve charge-discharge cycle characteristic.
In a second aspect of the present invention, a particulate Li-transition metal composite oxide having an average particle size of not less than 10 xcexcm is used as a positive electrode active material, and a mixture of a conductive material having a large particle size and a conductive material having a small particle size is used alongside, thereby to improve safety and charge-discharge cycle characteristic of the battery.
In a third aspect of the present invention, graphitized carbon having a specific surface area of not more than 2.0 m2/g, a spacing of lattice planes (d002) of not more than 0.3380 nm and a crystallite size in the c-axis direction (Lc) of not less than 30 nm is used as a negative electrode active material, and a mixed solvent for an electrolyte, which comprises ethylene carbonate, propylene carbonate, dimethyl carbonate and at least one member selected from the group consisting of diethyl carbonate and ethylmethyl carbonate is used, thereby to improve charge-discharge cycle characteristic.
In a fourth aspect of the present invention, a particulate Li-transition metal composite oxide having an average particle size of not less than 10 xcexcm, wherein [20/(specific surface areaxc3x97average particle size) ]=7-9, is used as a positive electrode active material, a mixture of a conductive material having a large particle size and a conductive material having a small particle size is used as a conductive material, graphitized carbon having a specific surface area of not more than 2.0 m2/g, a spacing of lattice planes (d002) of not more than 0.3380 nm and a crystallite size in the c-axis direction (Lc) of not less than 30 nm is used as a negative electrode active material, and a mixed solvent for an electrolyte, which comprises ethylene carbonate, propylene carbonate, dimethyl carbonate and at least one member selected from the group consisting of diethyl carbonate and ethylmethyl carbonate is used, thereby to improve charge-discharge cycle characteristic, storage characteristic and low temperature characteristic.
In a fifth aspect of the present invention, a mixed solvent for an electrolyte is used, which comprises ethylene carbonate in a proportion of 4% by volume-10% by volume, propylene carbonate in a proportion of 10% by volume-17% by volume, dimethyl carbonate in a proportion of 30% by volume-40% by volume, and at least one member selected from the group consisting of diethyl carbonate and ethylmethyl carbonate in a proportion of 40% by volumexe2x80x9450% by volume, thereby to inhibit reduction of discharge capacity and discharge voltage at xe2x88x9220xc2x0 C. or below.
In a sixth aspect of the present invention, a lithium ion secondary battery shows, upon 1 C discharge at xe2x88x9220xc2x0 C., (i) a backslash discharge curve without a minimum value, or (ii) a discharge curve with a minimum value and a maximum value appearing in the discharge capacity rate increasing direction, or (iii) a discharge curve with a first maximum value, a minimum value and a second maximum value appearing in the discharge capacity rate increasing direction, these three curves plotted in the coordinate where the axis of abscissa shows a discharge capacity rate based on a discharge capacity (100%) upon 1 C discharge at 20xc2x0 C., and the axis of ordinate shows a discharge voltage, wherein, in the case of the curve of (ii), a difference between the minimum value and the maximum value is not more than 0.1 V, and a difference between the minimum value and a discharge voltage, when the discharge capacity rate is 0%, is not more than 0.3 V, and, in the case of the curve of (iii), a difference between the minimum value and the second maximum value is not more than 0.1 V, and a difference between the minimum value and a discharge voltage, when the discharge capacity rate is 0%, is not more than 0.3 V, which battery showing a discharge capacity upon 1 C discharge at xe2x88x9220xc2x0 C. of not less than 60% of the discharge capacity upon discharge at 20xc2x0 C.